5种回到顶部的写法从实现到增强 - 小火柴的蓝色理想 - 博客园

前面的话

　　本文先详细介绍回到顶部的5种写法，然后对其实现功能增加，最后得到最终实现

写法

【1】锚点

　　使用锚点链接是一种简单的返回顶部的功能实现。该实现主要在页面顶部放置一个指定名称的锚点链接，然后在页面下方放置一个返回到该锚点的链接，用户点击该链接即可返回到该锚点所在的顶部位置

　　[注意]关于锚点的详细信息移步至此

<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
    <span><</span><span>div </span><span>id</span><span>="topAnchor"</span><span>></</span><span>div</span><span>></span>
    <span><</span><span>a </span><span>href</span><span>="#topAnchor"</span><span> style</span><span>="position:fixed;right:0;bottom:0"</span><span>></span>回到顶部<span></</span><span>a</span><span>></span>
<span></</span><span>body</span><span>></span>

【2】scrollTop

　　scrollTop属性表示被隐藏在内容区域上方的像素数。元素未滚动时，scrollTop的值为0，如果元素被垂直滚动了，scrollTop的值大于0，且表示元素上方不可见内容的像素宽度

　　由于scrollTop是可写的，可以利用scrollTop来实现回到顶部的功能

　　[注意]关于页面的scrollTop的兼容问题详细内容移步至此

<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
    <span><</span><span>button </span><span>id</span><span>="test"</span><span> style</span><span>="position:fixed;right:0;bottom:0"</span><span>></span>回到顶部<span></</span><span>button</span><span>></span>
    <span><</span><span>script</span><span>></span><span>
        test.onclick </span><span>=</span> <span>function</span><span>(){
            document.body.scrollTop </span><span>=</span><span> document.documentElement.scrollTop </span><span>=</span> <span>0</span><span>;
        }
    </span><span></</span><span>script</span><span>></span>
<span></</span><span>body</span><span>></span>

【3】scrollTo()

　　scrollTo(x,y)方法滚动当前window中显示的文档，让文档中由坐标x和y指定的点位于显示区域的左上角

　　设置scrollTo(0,0)可以实现回到顶部的效果

<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
    <span><</span><span>button </span><span>id</span><span>="test"</span><span> style</span><span>="position:fixed;right:0;bottom:0"</span><span>></span>回到顶部<span></</span><span>button</span><span>></span>
    <span><</span><span>script</span><span>></span><span>
        test.onclick </span><span>=</span> <span>function</span><span>(){
            scrollTo(</span><span>0</span><span>,</span><span>0</span><span>);
        }
    </span><span></</span><span>script</span><span>></span>
<span></</span><span>body</span><span>></span>

【4】scrollBy()

　　scrollBy(x,y)方法滚动当前window中显示的文档，x和y指定滚动的相对量

　　只要把当前页面的滚动长度作为参数，逆向滚动，则可以实现回到顶部的效果

<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
    <span><</span><span>button </span><span>id</span><span>="test"</span><span> style</span><span>="position:fixed;right:0;bottom:0"</span><span>></span>回到顶部<span></</span><span>button</span><span>></span>
    <span><</span><span>script</span><span>></span><span>
        test.onclick </span><span>=</span> <span>function</span><span>(){
            </span><span>var</span><span> top </span><span>=</span><span> document.body.scrollTop </span><span>||</span><span> document.documentElement.scrollTop
            scrollBy(</span><span>0</span><span>,</span><span>-</span><span>top);
        }
    </span><span></</span><span>script</span><span>></span>
<span></</span><span>body</span><span>></span>

【5】scrollIntoView()

　　Element.scrollIntoView方法滚动当前元素，进入浏览器的可见区域　

　　该方法可以接受一个布尔值作为参数。如果为true，表示元素的顶部与当前区域的可见部分的顶部对齐（前提是当前区域可滚动）；如果为false，表示元素的底部与当前区域的可见部分的尾部对齐（前提是当前区域可滚动）。如果没有提供该参数，默认为true

　　使用该方法的原理与使用锚点的原理类似，在页面最上方设置目标元素，当页面滚动时，目标元素被滚动到页面区域以外，点击回到顶部按钮，使目标元素重新回到原来位置，则达到预期效果

<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
    <span><</span><span>div </span><span>id</span><span>="target"</span><span>></</span><span>div</span><span>></span>
    <span><</span><span>button </span><span>id</span><span>="test"</span><span> style</span><span>="position:fixed;right:0;bottom:0"</span><span>></span>回到顶部<span></</span><span>button</span><span>></span>
    <span><</span><span>script</span><span>></span><span>
        test.onclick </span><span>=</span> <span>function</span><span>(){
            target.scrollIntoView();
        }
    </span><span></</span><span>script</span><span>></span>
<span></</span><span>body</span><span>></span>

增强

　　下面对回到顶部的功能进行增强

【1】显示增强

　　使用CSS画图，将“回到顶部”变成可视化的图形(如果兼容IE8-浏览器，则用图片代替)

　　使用CSS伪元素及伪类hover效果，当鼠标移动到该元素上时，显示回到顶部的文字，移出时不显示　　

<span><</span><span>style</span><span>></span><span>
.box</span><span>{</span><span>
    position</span><span>:</span><span>fixed</span><span>;</span><span>
    right</span><span>:</span><span>10px</span><span>;</span><span>
    bottom</span><span>:</span><span> 10px</span><span>;</span><span>
    height</span><span>:</span><span>30px</span><span>;</span><span>
    width</span><span>:</span><span> 50px</span><span>;</span><span>    
    text-align</span><span>:</span><span>center</span><span>;</span><span>
    padding-top</span><span>:</span><span>20px</span><span>;</span><span>    
    background-color</span><span>:</span><span> lightblue</span><span>;</span><span>
    border-radius</span><span>:</span><span> 20%</span><span>;</span><span>
    overflow</span><span>:</span><span> hidden</span><span>;</span>
<span>}</span><span>
.box:hover:before</span><span>{</span><span>
    top</span><span>:</span><span>50%
</span><span>}</span><span>
.box:hover .box-in</span><span>{</span><span>
    visibility</span><span>:</span><span> hidden</span><span>;</span>
<span>}</span><span>
.box:before</span><span>{</span><span>
    position</span><span>:</span><span> absolute</span><span>;</span><span>
    top</span><span>:</span><span> -50%</span><span>;</span><span>
    left</span><span>:</span><span> 50%</span><span>;</span><span>
    transform</span><span>:</span><span> translate(-50%,-50%)</span><span>;</span><span>
    content</span><span>:</span><span>'回到顶部'</span><span>;</span><span>
    width</span><span>:</span><span> 40px</span><span>;</span><span>
    color</span><span>:</span><span>peru</span><span>;</span><span>
    font-weight</span><span>:</span><span>bold</span><span>;</span>

<span>}</span><span>    
.box-in</span><span>{</span><span>
    visibility</span><span>:</span><span> visible</span><span>;</span><span>
    display</span><span>:</span><span>inline-block</span><span>;</span><span>
    height</span><span>:</span><span>20px</span><span>;</span><span>
    width</span><span>:</span><span> 20px</span><span>;</span><span>
    border</span><span>:</span><span> 3px solid black</span><span>;</span><span>
    border-color</span><span>:</span><span> white transparent transparent white</span><span>;</span><span>
    transform</span><span>:</span><span>rotate(45deg)</span><span>;</span>
<span>}</span>
<span></</span><span>style</span><span>></span>

<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
<span><</span><span>div </span><span>id</span><span>="box"</span><span> class</span><span>="box"</span><span>></span>
    <span><</span><span>div </span><span>class</span><span>="box-in"</span><span>></</span><span>div</span><span>></span>
<span></</span><span>div</span><span>></span>    
<span></</span><span>body</span><span>></span>

【2】动画增强

　　为回到顶部增加动画效果，滚动条以一定的速度回滚到顶部

　　动画有两种：一种是CSS动画，需要有样式变化配合transition；一种是javascript动画，使用定时器来实现　　

　　在上面的5种实现中，scrollTop、scrollTo()和scrollBy()方法可以增加动画，且由于无样式变化，只能增加javascript动画

　　定时器又有setInterval、setTimeout和requestAnimationFrame这三种可以使用，下面使用性能最好的定时器requestAnimationFrame来实现

　　[注意]IE9-浏览器不支持该方法，可以使用setTimeout来兼容

　　1、增加scrollTop的动画效果

　　使用定时器，将scrollTop的值每次减少50，直到减少到0，则动画完毕

<span><</span><span>script</span><span>></span>
<span>var</span><span> timer  </span><span>=</span> <span>null</span><span>;
box.onclick </span><span>=</span> <span>function</span><span>(){
    cancelAnimationFrame(timer);
    timer </span><span>=</span><span> requestAnimationFrame(</span><span>function</span><span> fn(){
        </span><span>var</span><span> oTop </span><span>=</span><span> document.body.scrollTop </span><span>||</span><span> document.documentElement.scrollTop;
        </span><span>if</span><span>(oTop </span><span>></span> <span>0</span><span>){
            document.body.scrollTop </span><span>=</span><span> document.documentElement.scrollTop </span><span>=</span><span> oTop </span><span>-</span> <span>50</span><span>;
            timer </span><span>=</span><span> requestAnimationFrame(fn);
        }</span><span>else</span><span>{
            cancelAnimationFrame(timer);
        }    
    });
}
</span><span></</span><span>script</span><span>></span>

　【时间版运动】

　　但是，上面的代码有一个问题，就是当页面内容较多时，回到顶部的动画效果将持续很长时间。因此，使用时间版的运动更为合适，假设回到顶部的动画效果共运动500ms，则代码如下所示

<span><</span><span>body </span><span>style</span><span>="height: 2000px;"</span><span>></span>
<span><</span><span>button </span><span>id</span><span>="test"</span><span> style</span><span>="position:fixed;right:10px;bottom:10px;"</span><span>></span>回到顶部<span></</span><span>button</span><span>></span>
<span><</span><span>script</span><span>></span>
<span>var</span><span> timer  </span><span>=</span> <span>null</span><span>;
test.onclick </span><span>=</span> <span>function</span><span>(){
    cancelAnimationFrame(timer);
    </span><span>//</span><span>获取当前毫秒数</span>
    <span>var</span><span> startTime </span><span>=</span> <span>+</span><span>new</span><span> Date();     
    </span><span>//</span><span>获取当前页面的滚动高度</span>
    <span>var</span><span> b </span><span>=</span><span> document.body.scrollTop </span><span>||</span><span> document.documentElement.scrollTop;
    </span><span>var</span><span> d </span><span>=</span> <span>500</span><span>;
    </span><span>var</span><span> c </span><span>=</span><span> b;
    timer </span><span>=</span><span> requestAnimationFrame(</span><span>function</span><span> func(){
        </span><span>var</span><span> t </span><span>=</span><span> d </span><span>-</span><span> Math.max(</span><span>0</span><span>,startTime </span><span>-</span><span> (</span><span>+</span><span>new</span><span> Date()) </span><span>+</span><span> d);
        document.documentElement.scrollTop </span><span>=</span><span> document.body.scrollTop </span><span>=</span><span> t </span><span>*</span><span> (</span><span>-</span><span>c) </span><span>/</span><span> d </span><span>+</span><span> b;
        timer </span><span>=</span><span> requestAnimationFrame(func);
        </span><span>if</span><span>(t </span><span>==</span><span> d){
          cancelAnimationFrame(timer);
        }
    });
}
</span><span></</span><span>script</span><span>></span>
<span></</span><span>body</span><span>></span>

　　2、增加scrollTo()动画效果

　　将scrollTo(x,y)中的y参数通过scrollTop值获取，每次减少50，直到减少到0，则动画完毕

<span><</span><span>script</span><span>></span>
<span>var</span><span> timer  </span><span>=</span> <span>null</span><span>;
box.onclick </span><span>=</span> <span>function</span><span>(){
    cancelAnimationFrame(timer);
    timer </span><span>=</span><span> requestAnimationFrame(</span><span>function</span><span> fn(){
        </span><span>var</span><span> oTop </span><span>=</span><span> document.body.scrollTop </span><span>||</span><span> document.documentElement.scrollTop;
        </span><span>if</span><span>(oTop </span><span>></span> <span>0</span><span>){
            scrollTo(</span><span>0</span><span>,oTop</span><span>-</span><span>50</span><span>);
            timer </span><span>=</span><span> requestAnimationFrame(fn);
        }</span><span>else</span><span>{
            cancelAnimationFrame(timer);
        }    
    });
}
</span><span></</span><span>script</span><span>></span>

　　3、增加scrollBy()动画效果

　　将scrollBy(x,y)中的y参数设置为-50，直到scrollTop为0，则回滚停止

<span><</span><span>script</span><span>></span>
<span>var</span><span> timer  </span><span>=</span> <span>null</span><span>;
box.onclick </span><span>=</span> <span>function</span><span>(){
    cancelAnimationFrame(timer);
    timer </span><span>=</span><span> requestAnimationFrame(</span><span>function</span><span> fn(){
        </span><span>var</span><span> oTop </span><span>=</span><span> document.body.scrollTop </span><span>||</span><span> document.documentElement.scrollTop;
        </span><span>if</span><span>(oTop </span><span>></span> <span>0</span><span>){
            scrollBy(</span><span>0</span><span>,</span><span>-</span><span>50</span><span>);
            timer </span><span>=</span><span> requestAnimationFrame(fn);
        }</span><span>else</span><span>{
            cancelAnimationFrame(timer);
        }    
    });
}
</span><span></</span><span>script</span><span>></span>

实现

　　由于scrollTop、scrollBy()和scrollTo()方法，都以scrollTop值是否减少为0作为动画停止的参照，且三个动画的原理和实现都基本相似，性能也相似。最终，以最常用的scrollTop属性实现动画增强效果

　　当然，如果觉得500ms的时间不合适，可以根据实际情况进行调整

<span><!</span><span>DOCTYPE html</span><span>></span>
<span><</span><span>html </span><span>lang</span><span>="en"</span><span>></span>
<span><</span><span>head</span><span>></span>
<span><</span><span>meta </span><span>charset</span><span>="UTF-8"</span><span>></span>
<span><</span><span>title</span><span>></span>Document<span></</span><span>title</span><span>></span>
<span><</span><span>style</span><span>></span><span>
.box</span><span>{</span><span>
    position</span><span>:</span><span>fixed</span><span>;</span><span>
    right</span><span>:</span><span>10px</span><span>;</span><span>
    bottom</span><span>:</span><span> 10px</span><span>;</span><span>
    height</span><span>:</span><span>30px</span><span>;</span><span>
    width</span><span>:</span><span> 50px</span><span>;</span><span>    
    text-align</span><span>:</span><span>center</span><span>;</span><span>
    padding-top</span><span>:</span><span>20px</span><span>;</span><span>    
    background-color</span><span>:</span><span> lightblue</span><span>;</span><span>
    border-radius</span><span>:</span><span> 20%</span><span>;</span><span>
    overflow</span><span>:</span><span> hidden</span><span>;</span>
<span>}</span><span>
.box:hover:before</span><span>{</span><span>
    top</span><span>:</span><span>50%
</span><span>}</span><span>
.box:hover .box-in</span><span>{</span><span>
    visibility</span><span>:</span><span> hidden</span><span>;</span>
<span>}</span><span>
.box:before</span><span>{</span><span>
    position</span><span>:</span><span> absolute</span><span>;</span><span>
    top</span><span>:</span><span> -50%</span><span>;</span><span>
    left</span><span>:</span><span> 50%</span><span>;</span><span>
    transform</span><span>:</span><span> translate(-50%,-50%)</span><span>;</span><span>
    content</span><span>:</span><span>'回到顶部'</span><span>;</span><span>
    width</span><span>:</span><span> 40px</span><span>;</span><span>
    color</span><span>:</span><span>peru</span><span>;</span><span>
    font-weight</span><span>:</span><span>bold</span><span>;</span>

<span>}</span><span>    
.box-in</span><span>{</span><span>
    visibility</span><span>:</span><span> visible</span><span>;</span><span>
    display</span><span>:</span><span>inline-block</span><span>;</span><span>
    height</span><span>:</span><span>20px</span><span>;</span><span>
    width</span><span>:</span><span> 20px</span><span>;</span><span>
    border</span><span>:</span><span> 3px solid black</span><span>;</span><span>
    border-color</span><span>:</span><span> white transparent transparent white</span><span>;</span><span>
    transform</span><span>:</span><span>rotate(45deg)</span><span>;</span>
<span>}</span>
<span></</span><span>style</span><span>></span>
<span></</span><span>head</span><span>></span>
<span><</span><span>body </span><span>style</span><span>="height:2000px;"</span><span>></span>
<span><</span><span>div </span><span>id</span><span>="box"</span><span> class</span><span>="box"</span><span>></span>
    <span><</span><span>div </span><span>class</span><span>="box-in"</span><span>></</span><span>div</span><span>></span>
<span></</span><span>div</span><span>></span>    
<span><</span><span>script</span><span>></span>
<span>var</span><span> timer  </span><span>=</span> <span>null</span><span>;
box.onclick </span><span>=</span> <span>function</span><span>(){
    cancelAnimationFrame(timer);
    </span><span>//</span><span>获取当前毫秒数</span>
    <span>var</span><span> startTime </span><span>=</span> <span>+</span><span>new</span><span> Date();     
    </span><span>//</span><span>获取当前页面的滚动高度</span>
    <span>var</span><span> b </span><span>=</span><span> document.body.scrollTop </span><span>||</span><span> document.documentElement.scrollTop;
    </span><span>var</span><span> d </span><span>=</span> <span>500</span><span>;
    </span><span>var</span><span> c </span><span>=</span><span> b;
    timer </span><span>=</span><span> requestAnimationFrame(</span><span>function</span><span> func(){
        </span><span>var</span><span> t </span><span>=</span><span> d </span><span>-</span><span> Math.max(</span><span>0</span><span>,startTime </span><span>-</span><span> (</span><span>+</span><span>new</span><span> Date()) </span><span>+</span><span> d);
        document.documentElement.scrollTop </span><span>=</span><span> document.body.scrollTop </span><span>=</span><span> t </span><span>*</span><span> (</span><span>-</span><span>c) </span><span>/</span><span> d </span><span>+</span><span> b;
        timer </span><span>=</span><span> requestAnimationFrame(func);
        </span><span>if</span><span>(t </span><span>==</span><span> d){
          cancelAnimationFrame(timer);
        }
    });
}
</span><span></</span><span>script</span><span>></span>
<span></</span><span>body</span><span>></span>
<span></</span><span>html</span><span>></span>

　　欢迎交流